As it is being transcribed, hnRNA is combined with a set of proteins to form an hnRNP complex. These proteins have a dramatic influence on the structure of the RNA molecules to which they are bound, and as a consequence they almost certainly influence the metabolism of the nuclear mRNA precursors. The work proposed here is directed toward a detailed understanding of the interactions between hnRNA and the structural proteins with which it associates. Based on hydrodynamic and electron microscopic studies, we have recently postulated a model for the basic architecture of hnRNP complexes of the brine shrimp, Artemia salina. Although the protein composition of these complexes is much simpler than the mammalian hnRNP, it is likely that they both share the same basic structure. We now intend to test and refine this hypothesis by turning our major efforts toward studies of the more complex mammalian hnRNP. We specifically aim to purify the major mammalian hnRNP proteins and, using the protocols developed in our studies of the major A. salina hnRNP protein, HD40, characterize their interactions with RNA and each other, and examine the structures of the complexes that are formed. In these studies we will reply primarily on physical methods, particularly spectroscopy, and electron microscopy. The electron microscopy studies in particular should be greatly facilitated by our experience with HD40 and A. salina hnRNP. In addition, since HD40 provides such a wonderful system for studying the basic properties of these complexes, I plan to continue our studies of HD40 through thermodynamic measurements of the protein-protein and protein-RNA interactions, digestion experiments, and high resolution electron microscopy and image reconstitution.